Thermal and chemical stratification of urban ponds: Are they 'completely mixed reactors'?

We examined whether relatively small urban ponds behave as 'completely mixed reactors' by measuring the stability and vertical heterogeneity of water column parameters. To do this, we measured profiles of water temperature and conductivity from the pond surface to the water-sediment interface twice during summer 2009 in 45 stormwater ponds located in the residential landscape of southern Ontario. These data were used to calculate water column stability indices and examine the relative importance of temperature and conductivity in controlling water column stratification. We also measured concentrations of dissolved oxygen (DO), dissolved organic carbon (DOC), total dissolved phosphorus (TDP),total dissolved nitrogen (TDN), particulate phosphorus and nitrogen (seston P and N), chlorophyll a (Chl-a), and total suspended solids (TSS) in surface and bottom waters. Despite their shallow depth (0.50-2.8 m), most ponds were stratified on the day of sampling and had relatively stable water columns both in June and August. Temperature differences rather than conductivity differences were better related to water column stability. Despite relatively stable water columns, vertical differences were not found for most water chemistry parameters, suggesting either recent mixing or relatively slow biogeochemical processing. Top-bottom differences were observed in June and August for DO and in June for TDN, seston N and TSS, reflecting a combination of processes influencing water column chemistry.